Tarique Rahman, the fugitive son of former Bangladesh Prime Minister Khaleda Zia, was sentenced to life and 19 others were given death sentence by a court on Wednesday over the 2004 grenade attack that killed 24 people and injured 500 others, including Prime Minister Sheikh Hasina.
The attack on an Awami League rally on August 21, 2004 targeted Ms Hasina, who was the opposition leader at that time. Ms Hasina survived the attack with a partial hearing loss.
The verdict comes ahead of the election in December. Mr Zia's Bangladesh Nationalist Party (BNP) had boycotted the 2014 election.
Security was tightened in the capital as the accused were brought to the court.
Rahman, 50, was tried in absentia with the court declaring him a "fugitive". He now lives in London where he is believed to have sought asylum though the British authorities have declined to reveal his immigration status.
He leads the main opposition Bangladesh Nationalist Party (BNP) from exile after Zia was jailed in February for five years for corruption.
Judge Shahed Nuruddin of Dhaka's fast track Tribunal pronounced the judgment ordering Rahman to be sent to prison for life along with 18 others.
Former state minister for home Lutfozzaman Babar is among 19 people who were sentenced to death.
Rahman, two former ministers including Babar and former top police and intelligence officials of the then BNP-led four-party alliance government were among 49 accused in the cases.
The judge made 12-point observations on the background, motive and consequences of the attack.
Investigations found an influential quarter of the then BNP-led government, including Rahman, masterminded and sponsored the attackers -- the operatives of militant Harkatul Jihad al Islami (HuJI).
Investigators said Ms Hasina was the main target of the attack. Hasina was injured in the attack while party's women front chief and former president Zillur Rahman's wife Ivy Rahman were among the dead.
Thirty one of the 49 convicts were present in the court on Wednesday while others were sentenced in absentia.
"We are taking steps to bring back the fugitive convicts," home minister Asaduzzaman Khan Kamal told reporters as the verdict was announced.
Those who were sentenced to death were fined Taka one lakh each. The High Court must confirm their death penalty after a mandatory review.
Other political figures, who were given life sentence, are ex-premier Zia's the then political adviser Haris Chowdhury and former BNP lawmaker Qazi Shah Mofazzal Hossain Kaikobad.